/*
 * Copyright (C) 2020 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.math;

import com.google.common.annotations.GwtIncompatible;

import java.math.BigDecimal;
import java.math.RoundingMode;

/**
 * A class for arithmetic on {@link BigDecimal} that is not covered by its built-in methods.
 *
 * @author Louis Wasserman
 * @since 30.0
 */
@GwtIncompatible
@ElementTypesAreNonnullByDefault
public class BigDecimalMath
{
    private BigDecimalMath()
    {
    }

    /**
     * Returns {@code x}, rounded to a {@code double} with the specified rounding mode. If {@code x}
     * is precisely representable as a {@code double}, its {@code double} value will be returned;
     * otherwise, the rounding will choose between the two nearest representable values with {@code
     * mode}.
     *
     * <p>For the case of {@link RoundingMode#HALF_DOWN}, {@code HALF_UP}, and {@code HALF_EVEN},
     * infinite {@code double} values are considered infinitely far away. For example, 2^2000 is not
     * representable as a double, but {@code roundToDouble(BigDecimal.valueOf(2).pow(2000), HALF_UP)}
     * will return {@code Double.MAX_VALUE}, not {@code Double.POSITIVE_INFINITY}.
     *
     * <p>For the case of {@link RoundingMode#HALF_EVEN}, this implementation uses the IEEE 754
     * default rounding mode: if the two nearest representable values are equally near, the one with
     * the least significant bit zero is chosen. (In such cases, both of the nearest representable
     * values are even integers; this method returns the one that is a multiple of a greater power of
     * two.)
     *
     * @throws ArithmeticException if {@code mode} is {@link RoundingMode#UNNECESSARY} and {@code x}
     *                             is not precisely representable as a {@code double}
     * @since 30.0
     */
    public static double roundToDouble(BigDecimal x, RoundingMode mode)
    {
        return BigDecimalToDoubleRounder.INSTANCE.roundToDouble(x, mode);
    }

    private static class BigDecimalToDoubleRounder extends ToDoubleRounder<BigDecimal>
    {
        static final BigDecimalToDoubleRounder INSTANCE = new BigDecimalToDoubleRounder();

        private BigDecimalToDoubleRounder()
        {
        }

        @Override
        double roundToDoubleArbitrarily(BigDecimal bigDecimal)
        {
            return bigDecimal.doubleValue();
        }

        @Override
        int sign(BigDecimal bigDecimal)
        {
            return bigDecimal.signum();
        }

        @Override
        BigDecimal toX(double d, RoundingMode mode)
        {
            return new BigDecimal(d);
        }

        @Override
        BigDecimal minus(BigDecimal a, BigDecimal b)
        {
            return a.subtract(b);
        }
    }
}
